The present invention relates to a lighting apparatus and a color measuring apparatus capable of adjusting the color temperature of a light source.
An example of a known lighting apparatus is shown in FIG. 16. The known lighting apparatus is provided with a main capacitor C1 which holds electric charge for a flash tube Xe; a DC power supply 1 serving as a power supply circuit block which charges the main capacitor C1 to a predetermined voltage; a reverse current preventing diode D1 which prevents the reverse flow of the electric charge from the main capacitor C1; a thyristor SCR2 serving as a switch element provided in a main discharge loop of the main capacitor C1; a commutation capacitor C2 which holds electric charge for forcing the electric current flowing through the main discharge loop to commutate; a thyristor SCR1 serving as a switch for discharging the commutation capacitor C2; resistors r1 and r2 for charging the commutation capacitor C2 at an initial level prior to light emission of the flash tube Xe; a trigger circuit Tr serving as a circuit for energizing the flash tube Xe; and a control circuit 2 which controls the start and forced stop of the light emission of the flash tube Xe.
In the lighting apparatus having the above-described construction, when the thyristor SCR2 is turned on when receiving a light emission start signal applied from the control circuit 2, simultaneously the trigger circuit Tr starts to operate. Consequently, the flash tube Xe starts emitting a light. When a time t1 shown in FIG. 17 is passed, the thyristor SCR1 is turned on in response to a light emission stop signal applied thereto from the control circuit 2. As a result, the electric charge in the commutation capacitor C2 reverse-biases the thyristor SCR2 between the anode and cathode thereof during the discharge time period T2 of the commutation capacitor C2 and the thyristor SCR2 is turned off. When a residual voltage of the main capacitor C1 becomes the same as the voltage charged on the commutation capacitor C2, the light emission of the flash tube Xe stops.
Thus, according to the above-described known lighting apparatus, in order to stop the light emission of the flash tube Xe, the thyristor SCR2 is reverse-biased between the anode and cathode thereof and turned off by turning on the thyristor SCR1 according to the light emission stop signal applied thereto from the control circuit 2. The flash tube Xe stops emitting a light when the residual voltage of the main capacitor C1 becomes balanced with the voltage charged on the commutation capacitor C2. For this reason, even after the thyristor SCR2 is turned off, electric current flows in the main discharge loop through the flash tube Xe, the commutation capacitor C2, and the thyristor SCR1. Therefore, an over-emission occurs as shown in FIG. 17. The over-emission period of the flash tube Xe greatly depends on the initial condition of the commutation capacitor C2, the temperature characteristics of the parts of the respective circuits and the light emission period. Accordingly, the over-emission period of the flash tube Xe varies every time as shown in FIG. 17.
That is, according to the above-described lighting apparatus, every time light is emitted, the over-emission period varies and as such, light emission quantities differ. Accordingly, it is difficult to make the color temperature of the light emitted by the flash tube Xe constant. It is also difficult to control the initial conditions and temperature characteristics of the main capacitor C1 and the commutation capacitor C2 so as to get a same over-emission period at every emission, namely, to get a same light emission quantity at every emission.
In addition, when reflectance or transmittance is measured based on light reflected by a sample or light which has passed therethrough by using the above-described lighting apparatus, S/N ratio is not preferable in a low reflectance wavelength band or a low transmittance wavelength band.
The color temperature of a light source is varied by discharged electric current and the temperature of a discharge tube. The discharged electric current can be made to be constant by appropriately constructing the power supply. But the temperature of the discharge tube rises as a result of repeated discharges and also rises if it repeats light emissions in a relatively short period. Therefore, it is difficult to keep the color temperature of the light source constant when the discharge tube is used as the light source.